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Undersea Jacuzzi may give life to Saturn’s icy moon

By Jacob Aron

Turn up the central heating

(Image: NASA/JPL/SSI)

New home on the market with basement Jacuzzi and easy commute to Saturn. Tiny sand grains gathered from Saturn’s rings reveal that Enceladus, already famous for its impressive water jets, is hiding a warm ocean deep beneath its surface. The finding suggests this moon is the only known place besides Earth with ongoing hydrothermal activity, and boosts the chances of finding life snuggled below its cold exterior.

NASA’s Cassini probe had already shown that Enceladus sprays plumes of dust and ice kilometres into the air. Combined with gravity data, this points to a subsurface ocean around 40 kilometres beneath the moon’s south pole. Such an ocean could be a boon to the search for alien life, but anything living there would need a source of energy, because the sun is too far away to provide much warmth.

Now painstaking detective work by Hsiang-Wen Hsu of the University of Colorado, Boulder, and his colleagues has made a connection between sand grains picked up by Cassini over a decade ago and the moon’s sea floor.

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Cassini isn’t able to scoop dust from Saturn’s rings directly, but instead samples material accelerated away from the planet by its magnetic field. Hsu traced some of these grains to Saturn’s E ring, which is thought to be created by Enceladus’s plumes.

Rocks under pressure

Cassini’s Cosmic Dust Analyzer instrument revealed that the dust consists of silica particles between 4 and 16 nanometres across – like sand, but much smaller. That means the grains couldn’t have formed from larger particles colliding within the rings, because then you would expect a greater range of sizes.

“It’s difficult to explain what we observed because we only see the particles in this narrow size range,” says Hsu.

The team wondered if chemical reactions between rock and water on Enceladus’s ocean floor could explain the finding. To investigate, they mixed water and silicon-rich rocks commonly found on asteroids and comets, then heated them under pressure for months to replicate conditions within Enceladus.

Out popped grains much like the ones Cassini had caught. What’s more, they provided a measure of the temperature inside Enceladus&colon; the right kind of particles only formed above 90 °C, suggesting the icy moon’s interior must be at least that warm.

“All this independent evidence points in one direction, that means these particles most likely form from hydrothermal interactions within Enceladus,” says Hsu.

Central heating

Hsu thinks the moon’s entire core is probably porous and filled with warm water that diffuses up to the subsurface ocean. But the source of all this heat is still a mystery. Any heat left over from Enceladus’s formation should have radiated away long ago. Tidal heating, generated by the squeeze of Saturn’s gravity, wouldn’t provide enough energy and chemical reactions or radioactive decay that could provide warmth should also have died out by now.

Enceladus may once have received a tidal heating boost from Saturn’s other moons that we are now seeing effects of, but the details aren’t clear, says Hsu.

There may be other reasons for the diminutive size of the grains seen by Cassini, says William McKinnon of Washington University in St Louis, but it’s a promising find. “If there is a source of energy, that’s one of the prerequisites for life,” he says. “If the story that they’ve put together is true, then the implications for what’s going on inside Enceladus are profound.”

Other icy moons are thought to harbour subsurface oceans, but the hydrothermal regions on Enceladus are particularly exciting because they closely match some found in the Atlantic, says Carolyn Porco, head of the Cassini imaging team and also at the University of Colorado, Boulder. Bacteria thrive at such sites on our planet. “When we can draw a connection between an environment on Earth and an environment on Enceladus, that just ups the ante for the possibility we might find life,” she says.

War of the watery worlds

Cassini is due to fly through one of the moon’s icy plumes later this year in search of molecular hydrogen, which might be produced in the reaction between the rock and water if it is occurring and could be a source of fuel for bacteria. But there is a limit to how much the probe can do. “All of these instruments were designed for another purpose,” says McKinnon. “We need to go back with instruments more specifically tuned to the Enceladus questions.”

Not everyone agrees Europa is the best way forward – although it does show signs of plumes, observations are less certain than those at Enceladus. “If the goal is to look for things having to do with astrobiology, Enceladus is a far better bet, but the train had left the station a long time ago to conduct a Europa mission,” says Porco.

McKinnon thinks Europa is worth the visit. Jupiter is much easier to reach than Saturn, and we’ve not had a dedicated probe there since the end of NASA’s Galileo mission in 2003, he says. “Given the greater gravity at Europa, it’s much harder to see plumes there. You could have Enceladus-like activity going on at Europa right now, and you just couldn’t tell because we don’t have a machine there.” In the war of the watery worlds, it looks like Europa might just beat Enceladus as the first place we find life beyond on Earth.